ERIC Hayman argues that wider adoption of 20mph speed limits across BCP is wrong (Letters 13 March). His primary argument is that diesel- and petrol-engined cars are less efficient driving at 20mph. However this argument is itself wrong.

At a constant speed, a modern car's fuel efficiency peaks at roughly 40mph. However on urban roads, constant speed is not possible. With a realistic speed profile, it is a 20mph limit that gives the best fuel efficiencies.

This is because acceleration becomes an increasingly large factor as speed increases. Typically on urban roads in a 20mph limit, acceleration accounts for 50 per cent of fuel use, at 30mph 80 per cent and at 40mph 90 per cent.

Levels of congestion and the number of junctions do have a big effect but studies show it would take a clear roundabout-free road and 'green lights all the way' before 30mph becomes more fuel efficient than 20mph.

Meaningful debate and consultation is surely required for wider adoption of 20mph limits. Sadly that probably won't happen as it seems a lot of folk have already made up their minds.

Dr Martin Rodger

Bloxworth Road,